14 NERVOUS SYSTEM AND SENSE ORGANS 



kind of smooth muscle fiber. As a result of their contraction the 

 opening into the sponge is lessened or closed. 



Wilson, 1910, describes membranes covering the subdermal 

 cavity and containing pores. This so-called membrane is composed 

 of an e.xternal portion and is believed to be syncytial. There are 

 two somewhat independent devices for the closure of pores, the pore 

 membrane and the pore canal sphincter. The closure of the pore 

 canals is dependent upon the sphincter-like band of cells on the 

 wall of the canal. These cells are in every way comparable to a 

 primitive form of smooth muscle-fiber. They are in contact with 

 the water passing into the canal and seem capable of direct stimu- 

 lation. The pore membrane is less muscle like and is perhaps of a 

 more primitive type. 



Parker, 1910 and 1919, considers the sponges as an important 

 group in illustrating the most primitive condition of the nervous 

 system of metazoans. Muscle cells the independent effectors, as 

 illustrated by the sphincters of sponges, were the first neuromuscu- 

 lar organs to appear. The special receptors in the way of sense- 

 cells were next to appear in certain coelenterates while in other 

 forms more complex, the adjuster or central organ was added. 



LITER.ATURE 

 Bidder, G. 



1896. The Collar-ceils of Heterocoela. Q. Jour. Mic. Sc. n.s. vol. 38, 

 pp. 9-43. pi. 2. 

 Lendenfeld, R. Von 



1885. Das Nervensystem der Spongien. Zool. Ang. Bd. 8, pp. 47-50. 2 fig. 



1887. Synocils, Sinnesorgane der Sponpien. Zool. Anz. bd. 10, pp. 142- 

 145. 2 text fig. 

 Minchin, E. A. 



1900. Sponges. A Treatise on Zoology edited bv E. R. Lanlcester. Part 

 2, eiiap. 3. 

 Parker, G. H. 



1909. The Origin of the Nervous System and its Appropriation of Ef- 

 fectors. Pop. Sc. Mo. vol. 75, pp. 56-64. 



1910. The Reactions of the Sponges with a Consideration of the Origin 

 of the Nervous System. Jour. Exp. Zool. vol. 8, pp. 1-41. 



1919. The Element-irv Nervous System. Monog. Exp. Biol. pp. 1-229. 

 53 illus. 

 Robertson, M. 



1911. The Division of the Collar Cells of the Calcarea Heterocoela. 

 Q. .Jour. Mic. Sc. n. s. Vol. 57. no. 226, pp. 129-139, pi. 19. 



